The invention relates generally to a modulating clutch for use in a power transmission system and more particularly to a drive line clutch having a magneto-rheological pilot operator.
The performance advantages of four-wheel vehicle drive systems are well recognized. Improved vehicle stability while traversing rain soaked or ice or snow covered highways, handling and control on gravel or uneven pavement and simply maintaining traction in off-road situations are all readily acknowledged benefits. Concomitant though less desirable characteristics of four-wheel drive systems relate to increased vehicle weight and increased drive line friction which result in reduced gas mileage. Such increased drive line friction results from the increased number of driven components and is especially significant in systems which are engaged full-time.
Such full-time four-wheel drive systems with direct coupling between the front and rear prop shafts represents one of the earlier four-wheel drive line configurations. A significant shortcoming of this arrangement involves a variation of a problem addressed by conventional rear differentials. As a vehicle negotiates a corner, the front wheels traverse paths of longer average radius and length than the rear wheels. Hence, the front drive line must rotate more rapidly during a turn. If unable to do this, due to the common drive between the front and rear drive lines, the drive shafts will wind up in opposite directions until a force produced by the stored (wind-up) torque in the drive shafts exceeds the frictional forces acting on the tires, the tires momentarily lose frictional contact, the drive lines unwind and the vehicle hops. Such operating conditions are both unacceptable to design engineers and unsettling to drivers. Installing a conventional differential assembly between the two drive lines such that they were capable of rotation at slightly different speeds (just as the left and right rear axles of a conventional vehicle are able to accommodate rotational speed differences) solved the wind up problem.
The placement of a differential assembly between the front and rear prop shafts of a four-wheel drive vehicle then created a problem similar to that of a conventional differential in a rear axle: the unwanted ability of one tire with less traction to spin while the other tire with traction receives no power. In four-wheel drive vehicles, this problem was addressed, for example, by the device disclosed in co-owned U.S. Pat. No. 4,718,303. Here, an electromagnetic clutch assembly progressively locked elements of a center differential assembly in order to ensure power delivery to all four vehicle wheels.
Next, the differential was eliminated and an electromagnetic clutch was utilized to directly transfer torque from the primary drive line to the secondary drive line as needed. See, for example, co-owned U.S. Pat. No. 5,407,024. With an increasingly sophisticated array of vehicle condition sensors and a concomitant increase in the sophistication of computer software, it becomes apparent that improvements are also possible in the structure and operation of the electromagnetic clutch. The present invention is directed to such a device which provides enhanced speed of operation and improved modulating control.